well i see what you mean with the "how" thing, but i dont think it is possible to give a proper answer to such an unspecific question.
building contextual menu modules in mac OS8.6+ are, to my knowlegde, documented in the carbon API, and to program your own you should get the carbon API, read the documentation, install a text editor and a C compiler, learn how to use them, then learn C++, then make up an idea , then see how you can implement it.
same goes with the apple script solution: install script editor, learn how to use it, learn how to script, build a script, put it into the CMM folder ...
the way how to get such things done lies somewhere between the rtfm and the principle of trial and error.
personally i have no clue about C++ and only basic knowwledge about applescript, so i wont be of much help here i guess.
otoh i believe that it should be possible for the average user to get it (for example) working to have a menu item which opens a blank simpletext document like asked above. probably 1.) create a simpletext document 2.) save as formular (or however thats called in english), 3.) move it into the CMM folder.
i have my default iText document in the apple menu ...